Golf ball cover

Abstract: A polymer composition and a golf ball employing a cover formed of that composition are disclosed. The polymer composition comprises a blend of a lithium ionomer resin with one or more sodium ionomer resins. When two sodium ionomer resins are used, one has a lower flexural modulus than the other and the ball employing such a cover has excellent spin characteristics.

Description: FIELD OF THE INVENTION This invention relates to golf balls and, more particularly, to golf ball cover compositions comprising blends of sodium ionomer resins and a lithium ionomer resin. Golf balls made with the cover composition of the present invention have been found to have good shear resistance, good initial velocity, and excellent spin rates. BACKGROUND OF THE INVENTION Generally, there are three types of golf balls in the market, namely one-piece, two-piece and wound. One-piece balls are molded from a homogeneous mass of material, while two-piece balls are made by molding a cover about a solid core. Wound balls are made by molding a cover about a wound core. A wound core is prepared by winding thin elastic thread about a center. Golf ball cores, whether wound or solid, typically measure from 1.4 to 1.6 inches (3.5 to 4.1 cm) in diameter. The cover is molded about the core to form a golf ball having the minimum United States Golf Association (USGA) specified diameter of 1.68 inches (4.3 cm). Typically, the cover has a thickness of about 0.04 inches (0.1 cm). Conventionally, both two-piece and wound golf balls are made by molding covers about cores in one of two ways: by injection molding fluid cover stock material around a core which is held in a retractable pin mold; or by compression molding preformed half-shells about the core. The preformed half-shells are formed by injecting fluid cover stock material into half-shell molds and solidifying the cover stock material into a half-shell shape. Balata was the standard cover stock material until the middle 1960's when E.I. DuPont de Nemours and Co. discovered a new species of resins known as ionomer resins. These resins are sold under the trademark SURLYN™ and, to a large extent, have replaced balata as a cover stock material. Chemically, these ionomer resins are a copolymer of an olefin and an alpha, beta ethylenically unsaturated carboxylic acid with 10-90% of the carboxylic acid groups being neutralized by a metal ion. See U.S. Patent No. 3,264,272. Today, the only commercially available ionomer resins are copolymers of ethylene and methacrylic or acrylic acid. These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization. Dunlop Rubber Company obtained the first patent on the use of Surlyn for the cover of a golf ball, see U.S. Patent No. 3,454,280 issued July 8, 1969. Since then, there have been a number of disclosures on the use of these ionomer resins in the cover composition of a golf ball. See, for example, U.S. Patent Nos. 3,819,768 issued June 25, 1974; 4,323,247 issued April 6, 1982; 4,526,375 issued July 2, 1985; 4,884,814 issued December 3, 1989; and 4,911,451 issued March 27, 1990. In November 1986 DuPont introduced a sodium and zinc ionomer resin having a low flexural modulus and suggested using and blending the same with other ionomer resins for making a golf ball cover. Golf ball covers made from these low flexural modulus ionomer resins have good shear resistance but very low velocity. In December of 1986, DuPont introduced a lithium ionomer resin which was a copolymer of ethylene and methacrylic acid. These lithium ionomer resins have a very high flexural modulus, typically about 60,000 psi (415 MPa). DuPont suggested that lithium ionomer resins could be used to produce a golf ball cover which would be more cut resistant and harder than a cover made with either sodium or zinc ionomer resins. DuPont also suggested that a golf ball having a cover made from a lithium ionomer resin would go farther, have a higher coefficient of restitution and be less prone to cutting (i.e., more durable) than a golf ball made from other known ionomer resins such as sodium and zinc ionomer resins and blends thereof. DuPont further suggested that lithium ionomer resins could be used in blends with other ionomer resins where they can impart better cut resistance to those other resins. The United States Golf Association (USGA) has promulgated a rule that no golf ball shall have an initial velocity that exceeds 255 feet (78 m) per second, i.e., 250 feet (76 m) per second with a 2% tolerance. Golf balls with covers made from ionomer resins with low flexural modulus are woefully below this maximum and, as should be appreciated, all golf ball manufacturers strive to come as close as possible to this limit. For many years, the ball of choice among advanced amateur players and tour professionals has been the balata covered wound ball. This ball is favored because it achieves spin rates sufficient to enable the player to more precisely control ball direction and distance, particularly on shorter approach shots. Balata covered wound balls, however, are expensive. In order to approximate the characteristics of balata covered wound balls at lower cost, the art has developed two piece balls of varying cover compositions. Despite numerous attempts to replicate the performance of balata covered wound balls, two piece balls generally suffer from low spin rates which makes them difficult to control near the greens. Also, two piece balls tend to have relatively poor click and feel. A need therefore exists for a two piece ball which has spin rates approximating those of balata covered wound balls. It has now been discovered that the spin rate of a two-piece golf ball which uses a cover formed of a blend of two or more sodium ionomers and a lithium ionomer approaches that of a wound balata ball. Thus viewed from one aspect the present invention provides a golf ball having a core and cover, wherein said cover is formed of about 10 to about 75 phr of a first sodium ionomer resin, said first sodium ionomer resin having a flexural modulus of about 20 MPa (2,800 psi), said first sodium ionomer resin is a terpolymer of about 5-50 weight percent of a softening comonomer of any of n- or iso-butyl acrylate, 4-30 weight percent of either acrylic acid or methacrylic acid in which 5-95 percent of the acid groups are neutralized by sodium, and the remainder is ethylene comonomer; about 2-10 phr of a second sodium ionomer resin, said second sodium ionomer resin having a flexural modulus of about 230 MPa (34,000 psi), said second sodium ionomer resin being a copolymer of about 95 to about 80 parts by weight ethylene and about 5 to about 20 parts by weight acrylic or methacrylic acid based on 100 parts by weight of copolymer, said second sodium ionomer resin having about 10 to about 90% of the acid groups neutralized by a sodium ion; and about 45 to about 85 phr of a lithium ionomer resin, said lithium ionomer resin having a flexural modulus of about 420 MPa (61,000 psi), said lithium ionomer resin being a copolymer of about 95 to about 80 parts by weight ethylene and about 5 to about 20 parts by weight acrylic or methacrylic acid based on 100 parts by weight of copolymer, said lithium ionomer resin having about 10 to about 90% of the acid group neutralized by a lithium ion. Viewed from a further aspect the invention provides a polymer composition comprising a blend of a lithium ionomer with one or more of a first sodium ionomer and a second sodium ionomer. The lithium ionomer is a copolymer comprising 95 to 80 parts by weight of ethylene and 5 to 20 parts by weight of acrylic or methacrylic acid, based on 100 parts by weight copolymer. The lithium ionomer resins useful in the invention typically have a very high flexural modulus which is above about 60,000 psi (415 MPa). More preferably, the lithium ionomer resin used in the present invention has a flexural modulus between about 60,000 and about 80,000 psi (415 and 550 MPa). Good results have been obtained with lithium ionomer resins having flexural modulii in the range of about 60,000 psi to about 70,000 psi (415 to 485 MPa). The most preferred lithium ionomer has a flexural modulus of about 420 MPa (61,000 psi). The lithium ionomer can be 45-85 phr, preferably 50-75 phr, most preferably 50 phr of the composition. In the polymer compositions of the invention, the first sodium ionomer preferably has a flexural modulus of nearly one tenth of the flexural modulus of the second sodium ionomer. The sodium ionomer is preferably a terpolymer comprising 5-50 weight percent of a softening comonomer such as n- or iso-butyl acrylate, 4-30 weight percent of either acrylic acid or methacrylic acid in which 5-95 percent of the acid groups are neutralized by sodium, and the remainder is ethylene comonomer. The first sodium ionomer, however, is preferably present in a greater amount than the second sodium ionomer. The first sodium ionomer can be 10-75 phr, preferably 45-75 phr, most preferably 45 phr of the composition, and the second sodium ionomer may be up to 15 phr, preferably 4-15 phr, most preferably 5 phr of the composition. Both the lithium and sodium ionomer resins preferably have 10% to 90% of their carboxylic acid groups neutralized by their respective metal ions. More preferably, both the lithium and sodium ionomer resins have their carboxylic acid groups neutralized by 35% to 65% by the metal ion. Preferably, the lithium and sodium ionomer resins have the same monocarboxylic acid, e.g. either methacrylic or acrylic acid. A golf ball having a cover comprising the polymer compositions described above also is provided. The flexural modulus is measured in accordance with A.S.T.M. Method D-790. Having briefly summarized the invention, the invention will now be described in detail by reference to the following specification and non-limiting examples. Unless otherwise specified, all temperatures are in degrees Celsius. DETAILED DESCRIPTION OF THE INVENTION To aid in the processing of fluid cover stock, it is conventional to use a plurality of ionomer resins to obtain the desired characteristics. Conventionally, ionomer resins with different melt flow indexes are employed to obtain the desired characteristics of the fluid cover stock. In order to adjust the characteristics of the fluid cover stock, other ionomer resins besides sodium and lithium can be employed. Good results have been obtained with lithium ionomer resins sold under the trade name SURLYN 8118, 7930 and 7940 by DuPont. Sodium ionomer resins sold by DuPont under the name SURLYN 8269, SURLYN 8320, SURLYN 8265, SURLYN 8120, and SURLYN 8660 also work well in the present invention. SURLYN 8118, 7930 and 7940 have flexural moduli of 61,000, 67,000 and 61,000 psi (420, 460 and 420 MPa) respectively. SURLYN 8269 and SURLYN 8320 each have a flexural modulus of 2,800 psi (20 MPa). SURLYN 8265 and SURLYN 8120 each have a flexural modulus of 7,100 psi (50 MPa), respectively. SURLYN 8660 has a flexural modulus 34,000 psi. SURLYN 8118, 7930 and 7940 have melt flow indexes of about 1.4, 1.8 and 2.6 g/10 min., respectively. SURLYN 8269, SURLYN 8320, SURLYN 8265 and SURLYN 8120 each have a melt flow index of about 0.9 g/10 min. SURLYN 8660 has a melt flow index of 10 g/10 min. Melt flow index is measured-in accordance with A.S.T.M. Test D 1238, condition E, procedure A. Preferably, the blend of ionomer resins used to make a cover of a golf ball in accordance with the present invention has a melt flow index between about 1 to about 4 g/10 min., more preferably about 1 to about 3 g/10 min. The combined amount of lithium ionomer resin and sodium ionomer resin used to make a cover in accordance with the present invention as herein described generally makes up at least about 90% by weight of the total weight of the golf ball cover and, preferably, at least about 95% by weight. Additional materials which may be included in the golf ball cover are other SURLYN resins; other resins such as polybutadiene, polyurethane and polyisoprene; whitening agents such as titanium dioxide; dyes; UV absorbers; optical brighteners; and other additives which are conventionally included in golf ball covers. The golf ball covers made in accordance with the present invention are made in a conventional manner by molding cover stock formed of the ionomer blends about a core. Molding is accomplished either by injection molding cover stock about a core or by compression molding preformed half-shells of the cover stock about a core. The preferred method is compression molding. Half-shells are made by injection molding a cover stock into a conventional half-shell mold in conventional manner at about 232°C (450°F) for about 40 seconds. The preformed half-shells are then placed about a core and the assembly is introduced into a compression molding machine. The compression molding machine is a hydraulic press having an upper and lower mold plate. As taught by U.S. Patent No. 4,508,309, such mold plate has half molds, each of which is registered with another half mold in the opposite mold plate. It has been found that a golf ball is formed with a cover in accordance with the present invention when the half-shells are compression molded about a core at about 127-149°C (260°F-300°F) for about 3 minutes. The molded balls are then cooled while still in the mold and removed when the cover is hard enough to be handled without deforming. After the balls have been molded, they undergo various conventional finishing operations such as buffing, painting and stamping. Preferably, the cover stock used to make golf ball covers in accordance with the present invention is a blend of lithium ionomer resin and sodium ionomer resins in the amounts specified hereinabove. Blending of the ionomer resins is accomplished in a conventional manner using conventional equipment. Good results have been obtained by mixing the ionomer resins in a solid, pelletized form and then placing the mix into a hopper which is used to feed the heated barrel of the injection molding machine. Further mixing is accomplished by a screw in the heated barrel. The injection molding machine is used either to make preformed half-shells for compression molding about a core or for molding flowable cover stock about a core using a retractable-pin mold. Such machines are conventional. These and other aspects of the present invention may be more fully understood by reference to the following examples. EXAMPLES 1-8 In accordance with the preferred embodiment of the invention, golf balls employing covers formed from blends of two or more sodium ionomers and a lithium ionomer are produced. Preferably, the sodium ionomers have very large differences in melt flow indices and flexural modulii. The sodium ionomers may be from 15-80 phr, preferably 50 phr of the composition. The lithium ionomer may be 20-80 phr of the composition, preferably 50 phr. Surlyn 8320 may be 10-50 phr, preferably 20-45 phr, most preferably 45 phr of the cover composition; Surlyn 8660 may be 0-10 phr, preferably 5 phr; and Surlyn 7940 may be 45-85 phr, preferably 50-75 phr, most preferably 50 phr by weight. Compositions of suitable cover blends in accordance with this embodiment of the invention are given in Examples 1-8 in Table 1. In accordance with the invention, it was found that balls using the cover blends of Examples 1-8 have spin rates intermediate between those of balata covered, wound balls and conventional two piece balls. Moreover, balls which employ covers formed of these terpolymers have click and feel superior to that of conventional two piece balls. The results are shown in Table 2. Example No. 1 2 3 4 5 6 7 8 Spin Rate 3403 3300 3300 3300 3300 3300 3400 3600 By comparison, the spin rates of balata covered wound balls (Titleist Tour 100) is 3800 and a conventional two piece ball (Titleist HVC) is 3000. It will be understood that each and every numerical value which appears in the claims herein is modified by the term "about" if the modifying term "about" does not appear in front of such numerical value. It will be understood that the claims are intended to cover all changes and modifications of the preferred embodiments of the invention, herein chosen for the purpose of illustration, which do not constitute a departure from the scope of the invention.

Claims: A polymer composition comprising a blend of a lithium ionomer with a first sodium ionomer and a second sodium ionomer, wherein said first sodium ionomer has a flexural modulus of about one-tenth or less than that of said second sodium ionomer. The polymer composition of claim 1 wherein said lithium ionomer is a copolymer comprising about 95 to 80 parts by weight of ethylene and about 5 to 20 parts by weight of acrylic or methacrylic acid, based on 100 parts by weight copolymer. The polymer composition of claim 1 or claim 2 wherein said lithium ionomer has a flexural modulus of about 420 MPa (61,000 psi). The polymer composition of any one of claims 1 to 3 wherein said lithium ionomer is about 45-85 phr by weight of said composition. The polymer composition of claim 4 wherein said lithium ionomer is about 50-75 phr of said composition. The polymer composition of claim 4 or claim 5 wherein said lithium ionomer is about 50 phr of said composition. A modification of a polymer composition as claimed in any one of claims 1 to 6 wherein said first sodium ionomer has a flexural modulus of about one tenth the flexural modulus of said second sodium ionomer. The polymer composition as claimed in any preceding claim wherein said first sodium ionomer has a flexural modulus of about 20 MPa (2,800 psi) and said second sodium ionomer has a flexural modulus of about 230 MPa (34,000 psi). The polymer composition of any one of claims 1 to 8 wherein said first sodium ionomer is present in a greater amount than said second sodium ionomer. The polymer composition of claim 9 wherein said first sodium ionomer is 20-45 phr of said composition, and said second sodium ionomer is 5 phr of said composition. The polymer composition of claim 10 wherein said first sodium ionomer is 45 phr of said composition, said second sodium ionomer is 5 phr of said composition and said lithium ionomer is 50 phr of said composition. A golf ball having a cover comprising a polymer composition as claimed in any one of claims 1 to 11. In a golf ball having a core and a cover, the improvement comprising a cover formed of about 10 to about 75 phr of a first sodium ionomer resin, said first sodium ionomer resin having a flexural modulus of between about 5 MPa and 140 MPa (1,000 psi and 20,000 psi), said first sodium ionomer resin is a terpolymer of about 5-50 weight percent of a softening comonomer of any of n- or iso-butyl acrylate, about 4-30 weight percent of either acrylic acid or methacrylic acid in which 5-95 percent of the acid groups are neutralized by sodium, and the remainder is ethylene comonomer; about 2-10 phr of a second sodium ionomer resin having a flexural modulus of about 230 MPa (34,000 psi), said second sodium ionomer resin being a copolymer of about 95 to about 80 parts by weight ethylene and about 5 to about 20 parts by weight acrylic or methacrylic acid based on 100 parts by weight of copolymer, said second sodium ionomer resin having about 10 to about 90% of the acid groups neutralized by a sodium ion; and about 45 to about 85 phr of a lithium ionomer resin, said lithium ionomer resin having a flexural modulus of between about 415 MPa and 550 MPa (60,000 psi and 80,000 psi), said lithium ionomer resin being a copolymer of about 95 to about 80 parts by weight ethylene and about 5 to about 20 parts by weight acrylic or methacrylic acid based on 100 parts by weight of copolymer, said lithium ionomer resin having about 10 to about 90% of the acid group neutralized by a lithium ion. The ball of claim 13, wherein said first sodium ionomer has a flexural modulus of about one-tenth or less than that of said second sodium ionomer. The ball of claims 13 or 14, wherein said lithium ionomer has a flexural modulus of about 420 MPa (61,000 psi). The ball of any one of claims 13 to 15, wherein said first sodium ionomer has a flexural modulus of about 20 MPa (2,800 psi). The ball of any one of claims 13 to 16, wherein said second sodium ionomer has a flexural modulus of about 230 MPa (34,000 psi). The ball of any one of claims 13 to 17, wherein said first sodium ionomer is present in an amount of about 45 phr. The ball of any one of claims 13 to 18, wherein said second sodium ionomer is present in an amount of about 5 phr. The ball of any one of claims 13 to 19, wherein said lithium ionomer is present in an amount of about 50 phr. The ball of any one of claims 13 to 17, wherein said first sodium ionomer is present in an amount of 10-50 phr, said second sodium ionomer is present in an amount of 4-8 phr, and said lithium ionomer is present in an amount of 50-75 phr. The ball of any one of claims 13 to 21, wherein said cover has a meet flow index from about 1 to about 4 g/10 min.

Inventors: STEFANI BARBARA, STEFANI, BARBARA
Applicants: ACUSHNET CO, ACUSHNET COMPANY
Application year: 1994
Main classification: C08L23
IPC codes: C08L23, A63B37